More than 3,000 plants containing essential oil have been found so far around the world. There are about 500 natural spices specified in the catalogues on the international market, about 200 of which are practically used as natural spices and sold as merchandise. At present, there are more than 120 natural spices that have been able to be produced in our country, such as cinnamon oil, peppermint oil, eucalyptus oil and turpentine, with the annual output standing in the front ranks of the world. Essential-oil plants, mainly seed plants, especially aromatic plants, are widely distributed in the nature of our country. There are 56 families, 136 genera and about 300 species of aromatic plants wild and cultivated in our country, with lemon oil, orange oil, limette oil and bergamot oil being imported much more than being exported for our country. They can be obtained by extraction or distillation recovery as a byproduct of the citrus juice processing. The main ingredient thereof, D-limonene, can be distilled out at atmospheric pressure without decomposition. Pomelo peel, accounting for 43% to 48% of the whole pomelo, in addition to containing such nutrients as water, vitamins and minerals that are essential to the human body, also contains a variety of non-nutritive physiologically active ingredients that are beneficial to the human health, such as flavonoids, limonoids, essential oils, natural pigments, and dietary fiber, which are more than those in the pomelo fruit and have higher health care and medicinal value.
The pomelo peel essential oil, containing many chemical ingredients, is a mixture and essentially consists of aliphatic compounds, aromatic compounds and terpenoids; the pomelo essential oil has a unique aroma, but can be neither chemically synthesized nor replaced by other citrus so far. Currently the pomelo peel essential oil is seldom sold on the market, while the demand for the pomelo peel essential oil has recently increased year by year, resulting in serious supply and demand imbalance. A squeezing method and a steam distillation method are often used for extraction of the citrus-peel essential oil, but both have some technical defects. With the squeezing method, the fresh pomelo peel is placed into a screw squeezer after being soaked with limewater, meanwhile an appropriate amount of spray water is used to elute the essential oil from the oil cell tissue, and then the essential oil is filtered with screen cloth, settles, and is separated by a high-speed centrifuge, so as to obtain a crude product of the citrus essential oil, which then undergoes solvent extraction and vacuum fractionation to get terpenoids eliminated, thereby producing the citrus essential oil without terpenoids; however, this method needs large workload of high-speed centrifugation in industry, and the oil will still contain some such impurities as pigments and mucilaginous substances after the high-speed centrifugation that are cumbersome to be separated. The steam distillation method, due to the high temperature process, will make some terpenoid ingredients oxidized and deteriorate, and have a low yield.
The Chinese invention patent 02152166.2 discloses a method of jasmine essential oil extraction by using the supercritical carbon dioxide. The Chinese patent 200510049888.9 discloses a process of citrus-peel essential oil extraction by squeezing with a sugarcane squeezer and then oil-water separation with a centrifuge. The Chinese invention patent 200810199137.9 discloses a method of essential oils extraction from the pomelo peel or orange peel by using the steam distillation. However, the methods involved in the above patents have the following problems: (1) The citrus essential oil is generally present in the oil cell layer thereof, having an epidermal thickness of 2.5-3 mm; the existing large-scale production of the citrus essential oil mostly adopts a water milling method to extract the oil-water mixture before separation. In terms of a pomelo, it has a thicker oil cell layer, a solid epidermis, and an irregular appearance, less likely to allow the large-scale application of the water milling method. (2) With the essential oil wrapped by a cell wall, simple squeezing or the water milling method is very difficult to get it extracted completely, and there are still some essential oils remaining in the oil cell layer and wrapped by fiber, resulting in a lower essential-oil extraction rate.